At some point, the kind of headphones you're using is a factor. That said, I think you could use an active solution of an op amp mixer with volume control on each input. A common IC like a NE5532 may provide enough output for your 'phones.

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It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

This schematic is from the Learn about Electronics website. I took the liberty of changing their 3-input mixer into the 2-input circuit you need.
The purpose of the second op amp is to maintain the same phase as the input signal. If you use it, put a resistor of 50-100 ohms between its output and the headphone jack. If you don't use it, make R6 that 50-100 ohm resistor.
The above description is for a mono output. If you want stereo, you'll need to make 2 of these circuits, one each for left and right channels.
You'll need a small power supply also. ±12VDC would work fine and be easy to build.

I neglected to say there's a few ways to combine these signals to the headphones.
*Left, right, and guitar on both sides of headphones.
*Left side, right side, and guitar on one side.
*Left side, right side, and guitar on both sides.
Not sure which you want, but each require a slightly different circuit based on the schematic above.

__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

That's a good question; maybe there is a hardware or software solution. I don't think any card I've owned could do it, but then again I've never tried. Anyhow, here's the new schemo. Mono so nothing needs duplicated for another channel.